1. Field of the Invention
The present invention relates to an oil separator for an engine which is configured to separate a liquid from a gas, and a personal watercraft comprising the oil separator.
2. Description of the Related Art
In recent years, four-cycle engines have been mounted in certain jet-propulsion personal watercraft provided with water jet pumps. In the engines, the internal crankcase pressure varies with the reciprocation of pistons. Accordingly, the engine is provided with a breather passage to reduce an increased internal pressure of the crankcase. The breather passage is connected to an air-intake system such as an air cleaner box to inhibit an excessive increase in the internal pressure of the crankcase.
Meanwhile, a chamber of the crankcase or an inside of a cylinder head connected to the chamber of the crankcase through a chain tunnel or the like, is full of oil in a mist state (oil mist). In order to inhibit a blow-by gas of the engine that contains the oil mist (hereinafter referred to as a liquid air-fuel mixture) from flowing into the air cleaner box through the breather pipe and the oil in the blow-by gas from flowing into the air-intake system, the breather passage of the engine is typically provided with an oil separator for separating the liquid air-fuel mixture into the oil and the gas.
The oil separator is configured to have an oil chamber having an inner space of a predetermined volume which is typically separated by a number of separating plates, thus forming a labyrinth structure. The blow-by gas with the oil mist that flows into the oil separator flows along passages formed by the separating plates provided within the inner space. While flowing along the passages, the oil contained in the blow-by gas collides with the separating plates and adheres to these plates. In this manner, the liquid air-fuel mixture flowing into the oil separator is separated into the gas and the oil. The gas is drawn into the air-intake system through the breather passage, and the oil is returned to an oil pan through an oil return passage.
In the conventional engine, as disclosed in Japanese Patent No. 3124743, an oil separator is provided at a connecting face between a crankcase and an oil pan, or a chamber independently provided within a cylinder head is configured to have a labyrinth-structured inner space which functions as an oil separator.
By the way, in the case of the jet-propulsion personal watercraft, a vertical dimension of the engine is inevitably limited because of restrictions such as a narrow inner space or center of gravity of a body. However, when the oil separator is provided at the above described location, the vertical dimension of the engine tends to be increased. Such an engine is difficult to mount in the narrow inner space within the body.
In the conventional oil separator having a labyrinth structure, the passages formed by a number of separating plates are relatively narrow, and are sometimes clogged with the separated oil. The oil that has clogged the passages is pushed into the breather passage by the blow-by gas and is carried into the air-intake system.
When the oil pan has a sufficient volume, the oil is also separated from the gas within an inner space of the oil pan. However, since the vertical dimension of the engine of the personal watercraft is limited as described above, a sufficient volume of the oil pan is difficult to gain. When the liquid air-fuel mixture that contains relatively large amount of oil might flow into the conventional oil separator having a labyrinth structure, the oil is not satisfactorily separated from the gas.